As modern commerce depends on reliable and cost-effective methods for delivering products from suppliers to users, the availability of durable and reliable highways, roads and other support surfaces for vehicles is vital for sustaining a modern economy. To provide better support surfaces, highways, roads, and sidewalks are commonly paved with a layer or mat of asphaltic concrete that is laid over the surface of the sub-base. Asphalt is preferred over cement to pour roads because it is less expensive and very durable. Asphalt can also be poured at night, which allows major roads to be shut down at the least busy of times for maintenance. Relative to road noise, asphalt is also quieter than cement, making it the better choice for roads.
Asphalt concrete is essentially a mixture of bitumen, as binder, with aggregate, in particular filler, sand, and stones. There are many different types of asphalts available and their characteristics can vary quite significantly. The design of asphalt mix for bituminous paving application is a complex process of selecting and proportioning materials to obtain the desired properties in the finished construction while minimize undesirable characteristics.
In evaluating and adjusting mix design, the aggregate gradation and the binder content in the final mix is balanced between the stability and durability requirements for the intended use. The final goal of mix design is to achieve a balance among all of the desired properties. Binders and various polymers have been investigated for reaching similar goals, and other modifications have been studied.
Unsaturated thermoplastic elastomers like styrene-butadiene-styrene (SBS) block copolymers are polymers used for asphalt modification. They enhance the elastic recovery capacities of asphalt and, therefore, its resistance to permanent deformations. However, unsaturated elastomeric polymers are quite expensive and are subjected to degradation when exposed to atmospheric agents and mechanical stress. Due to their fragility, they are typically used as virgin polymers. This can result in a significant cost increase for the product. While SBS is recognized for performance benefits, research has focused on most cost effective modifiers in exchange for sacrificing superior performance. In addition, many virgin polymers were used before in the modification of asphalt including polyethylene. However, these polymers are expensive in comparison with industrial wastes such as polyethylene wax.
Olefinic polymers also have been investigated for use as modifiers. They are available in large quantities with different mechanical properties and at low cost. Polyethylene (PE) and polypropylene (PP) are plastomers that have also been used. They bring a high rigidity (i.e., lack of elasticity, resistance to bending) to the product and significantly reduce deformations under traffic load. Due to their non-polar nature, PE and PP suffer from the drawback that they are almost completely immiscible with asphalt, and are thus limited in use. PE has a melting point in the range 110°-140° C. depending on the type of PE. PP has a higher melting point. The higher melting temperatures limit the complete mixing of the polyolefin with asphalt where the typical mixing temperatures are in the range 140°-160° C.
Conventional asphalts often do not retain sufficient elasticity in use and also exhibit a plasticity range which is too narrow for use in many modern applications such as road construction. The characteristics of road asphalts can be improved by incorporating into them an elastomeric-type polymer. There exists a wide variety of polymers that can be mixed with asphalt. Of these, SBS is a commonly used polymer in asphalt modification. The modified asphalts thus obtained are commonly referred to variously as bitumen/polymer binders or asphalt/polymer mixes. There is a need for a modification to hotmix asphalt concrete mixes that would increase the resistance to permanent deformation while maintaining or increasing the modulus of the mix at intermediate temperatures without affecting the binder properties significantly.
The bituminous binders, including of the bitumen/polymer type, which are employed at the present time in road applications, often do not have the optimum characteristics at low enough polymer concentrations to consistently meet the increasing structural and workability requirements imposed on roadway structures and their construction. In order to achieve a given level of modified asphalt performance, various polymers are added at some prescribed concentration. Current practice is to add the desired level of a single polymer, sometimes along with a reactant which promotes cross-linking of the polymer molecules until the desired asphalt properties are met. This reactant typically is sulfur in a form suitable for reacting.
Sulfur, especially “free” or “elemental” sulfur, is an abundant and inexpensive material. Elemental sulfur is a byproduct of non-sweet natural gas and petroleum processing. Sources of free sulfur include petroleum refineries and gas sweetening plants. Because of the quantity of sulfur extracted from natural gas and petroleum, many sulfur producers consider elemental sulfur a waste product.
Others have attempted to use waste sulfur as an expander or filler for asphalt binders, but only with limited success. These efforts have only been successful in incorporating a small amount of sulfur into the asphalt binder, typically only up to a few percent of the total composition.
Those skilled in the art understand that sulfur forms hydrogen sulfide (H2S) gas, which is toxic to humans, starting at around 150° C. At and above that temperature, free sulfur in hydrocarbon environments dehydrogenates hydrocarbons and forms hydrogen sulfide. Heating sulfur to high temperatures in the presence of oxygen forms sulfur dioxide, which is noxious to humans and is an air pollutant. It is desirable to find a combination of asphalt materials that are workable at temperatures below 150° C. for worker comfort and safety as well as being more to the environment.
It is also desirable to find commercial uses for elemental sulfur. Incorporating sulfur into commercial products can transform what many consider a potential “waste” product into a product that has practical value.
When added to bitumen at 140° C., sulfur is finely dispersed in bitumen as uniformly small particles; coagulation and settlement of sulfur particles become noticeable after a few hours. Therefore, the sulfur extended asphalt (SEA) mixtures can be produced directly in the mixing plant just before the laying of the asphalt mixture. One major concern in handling sulfur-asphalt mix is the fear of the evolution of hydrogen sulfide (H2S) during production and laying. This problem can be ameliorated by adding carbon or ash to sulfur. H2S evolution starts at temperatures higher than 150° C., so that the application at temperatures up to 150° C. avoids pollution and safety problems. However, H2S evolution starts well below 150° C., i.e. about 130° C., which is undesirable from an environmental perspective. Moreover, below 120° C., neither the reaction of the asphalt and sulfur nor the cross-linking of the SBS/sulfur blend could take place.
Besides performance and environmental issues associated with many types of asphalt modifiers, many of the polymers that are used to modify asphalt compositions are expensive and can be difficult to obtain in remote areas of the world.
A need exists for asphalt compositions with improved properties and reduced environmental and economic impact.